testutil: Also add callback errors to KVOperation

Change-Id: I1dfb168c46eb96b1cf4a4ccdf7d0c531f35346e5
Reviewed-on: http://review.couchbase.org/34567
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>